Null/centerpoints

Conditions (2.76) and (2.77) define independence of the design from rotation of coordinates. When selecting the null/centerpoints points (points in experimental center) take into consideration a check of lack of fit of the model, an estimate of experimental error and conditions of uniformity [37]. Centerpoints are created by setting all factors at their midpoints. In coded form, centerpoints fall at the all-zero level. The centerpoints act as a barometer of the variability in the system. All the necessary data for constructing the rotatable design matrix for k<7 are in Table 2.137. This kind of designing is called central, because all experimental points are symmetrical with reference to the experimental center. This is shown graphically for k=2 and k=3 in Fig. 2.40. 

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